1. Field of the Invention
The invention relates to data access of memories, and more particularly to data access of downgrade memories.
2. Description of the Related Art
Memories are divided into normal memories and downgrade memories. A memory comprises a plurality of memory cells for data storage. When a manufacturer produces a batch of memory chips, the manufacturer performs a quality test to determine the quality of the batch of memory chips. If the batch of memory chips cannot satisfy requirements of the quality test, the memory chips are then categorized into downgrade memory chips and sold in a price lower than that of normal memory chips. The low quality of the downgrade memory chips are due to defect memory cells of the downgrade memory chips. The defect memory cells cannot successfully store data, resulting in errors of data read from the downgrade memory chip.
Referring to FIG. 1A, a schematic diagram of block 150 of a downgrade memory is shown. The downgrade memory comprises a plurality of pages, each of the pages comprises a plurality of sectors, and each of the sectors comprises a plurality of memory cells for data storage. A sector 1 of a page 0 shown in FIG. 1A comprises a defect memory cell 161, a sector 2 of a page 1 shown in FIG. 1A comprises a defect memory cell 162, and a sector 0 of a page 2 shown in FIG. 1A comprises a defect memory cell 163. Because block 150 comprises a plurality of defect memory cells which cannot normally store data, a conventional controller of the downgrade memory marks block 150 as a defect block and does not use it to store data.
Block 150, however, comprises only three defect memory cells: 161, 162, and 163. Other memory cells of block 150 can still normally store data. Because the conventional controller directly avoids storing data in the entire block 150, the usable memory space of the downgrade memory is reduced, degrading the performance of the downgrade memory. Thus, a method for efficiently using downgrade memory to store data is required to increase the usable memory space of the downgrade memory.